The ability to measure the parameters of MOS devices is of great value, especially for technology development and accurate modeling of complementary MOS (CMOS) gate delays and leakage currents. The latter are important for circuit design applications and chip power management. Two examples of MOS characteristics are the physical gate length (Lpoly) and gate oxide thickness (Tox) for MOS field effect transistors (MOSFETs), and these characteristics are extremely important for determining the performance of MOS circuits.
Direct current (DC) characteristics of a MOSFET provide a direct technique for measuring threshold voltage, Vt, for the MOSFETs on a semiconductor. In CMOS technologies prior to generations having linewidths of about 0.18 pm, the effective length of the gate, Leff, was determined by extrapolation of the channel resistance of MOSFETs as a function of the designed gate length, Lmask. With the scaling of CMOS technologies, the scatter in the Leff measurement has become significant, and Leff has now been replaced by the measurement of physical gate length, Lpoly. The Lpoly is generally estimated from the resistance of the film constituting the gate or from the gate oxide capacitance for a constant thickness of the gate oxide, Tox. As the physical gate oxide thickness is reduced below 1.5 nm (such as 0.13 μm linewidth technology generation and beyond), the DC tunneling current interferes with the capacitance measurement.
Thus, what is needed are improved techniques for measuring characteristics of MOS devices.